In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
In an industrial gas turbine engine used for electrical power production, during periods of low electrical demand the engine is reduced in power. During periods of low electrical power demand, prior art power plants have a low power mode of 40% to 50% of peak load. At these low power modes, the engine efficiency is very low and thus the cost of electricity is higher than when the engine operates at full speed with the higher efficiency.
Industrial and marine gas turbine engines used today are shown in FIGS. 12-15. These designs suffer from several major issues that include low component (compressor and turbine) performance for high cycle pressure ratios or low part load component efficiencies or high CO (carbon monoxide) emissions at part load when equipped with low NOx combustors which limit the low power limit at which they are allowed to operate (referred to as the turn-down ratio).
FIG. 12 shows a single shaft IGT engine with a compressor 1 connected to a turbine 2 with a direct drive electric generator 3 on the compressor end. FIG. 13 shows a dual shaft IGT engine with a high spool shaft and a separate power turbine 4 that directly drives an electric generator 3. FIG. 14 shows a dual shaft aero derivative gas turbine engine with concentric spools in which a high pressure spool rotates around the low pressure spool, and where a separate low pressure shaft that directly drives an electric generator 3. FIG. 15 shows a three-shaft IGT engine with a low pressure spool rotating within a high pressure spool, and a separate power turbine 4 that directly drives an electric generator 3.
The configuration of FIG. 12 IGT engine is the most common for electric power generation and is limited by non-optimal shaft speeds for achieving high component efficiencies at high pressure ratios. The mass flow inlet and exit capacities are limited structurally by AN2 (last stage blade stress) and tip speeds that limit inlet and exit diameters due to high tip speed induced Mach number losses in the flow. Therefore for a given rotor speed, there is a maximum inlet diameter and corresponding flow capacity for the compressor and exit diameter and flow capacity for the turbine before the compressor and turbine component efficiencies start to drop off due to high Mach number losses.
Since there is a fixed maximum inlet flow at high pressure ratios on a single shaft, the rotor blades start to get very small in the high pressure region of the compressor flow path. The small blade height at a relatively high radius gives high losses due to clearance and leakage affects. High pressure ratio aircraft engines overcome this limitation by introduction of separate high pressure and low pressure shafts. The high pressure shaft turns at a faster speed allowing for smaller radius while still accomplishing a reasonable work per stage. An example for this is shown in FIG. 14, which is typical of an aero-derivative gas turbine engine used for electrical power production. The speed of the high pressure spool 5 is still limited by having a low speed shaft 6 inside the inner diameter (ID) of the high pressure shaft 5. This drives the high pressure shaft 5 flow path to a higher radius relative to what might otherwise be feasible, which thereby reduces the speed of the high pressure rotor, creating smaller radius blades which reduce the efficiency of the high pressure spool. FIG. 13 arrangement is similarly limited in achieving high component efficiencies at high pressure ratios as FIG. 12 since the entire compressor is on one shaft.
Turn down ratio is the ratio of the lowest power load at which a gas turbine engine can operate (and still achieve CO emissions below the pollution limit) divided by the full 100% load power. Today's gas turbines have a turn down ratio of around 40%. Some may be able to achieve 30%. Low part load operation requires a combination of low combustor exit temperatures and low inlet mass flows. Low CO emissions require a high enough combustor temperature to complete the combustion process. Since combustion temperature must be maintained to control CO emissions, the best way to reduce power is to reduce the inlet mass flow. Typical single shaft gas turbine engines use multiple stages of compressor variable guide vanes to reduced inlet mass flow. The limit for the compressor flow reduction is around 50% for single shaft constant rotor speed compressors as in FIG. 12. The FIG. 14 arrangement is similarly limited as the FIG. 12 arrangement in flow inlet mass flow reduction since the low pressure compressor runs at the constant speed of the generator. In industrial engine that drive electric generators, the turbine that drives the electric generator is set to operate at a constant speed such as 3,600 rpm for a 60 hertz engine in the USA or at 3,000 rpm for a 50 hertz engine in European countries.
The FIG. 15 arrangement is the most efficient option of the current configurations for IGT engines, but is not optimal because the low spool shaft 6 rotates within the high spool shaft 5, and thus a further reduction in the high spool radius cannot be achieved. In addition, if the speed of the low spool shaft 6 is reduced to reduce inlet mass flow, there is a mismatch of angle entering the LPT (Low Pressure Turbine) from the HPT (High Pressure Turbine) and mismatch of the flow angle exiting the LPT and entering the PT (Power Turbine) leading to inefficient turbine performance at part load.
FIG. 16 shows a prior art electric power plant that uses two prior art industrial gas turbine engines 111 that each drive an electric generator 112. In the prior art technology of today, each of these industrial engines 111 can produce up to 350 MW of output for a 60 hertz engine and up to 500 MW for a 50 hertz engine. When these prior art industrial engines 11 are to be replaced with new engines, two new industrial engines of equal power are required. When a new industrial gas turbine engine replaces an older industrial gas turbine engine in a combined cycle power plant, the turbine exhaust temperature of the new engine must be substantially the same as the turbine exhaust temperature of the older engine because the HRSG (Heat Recovery Steam Generator) of the combined cycle power plant would require significant structural changes to allow for the higher exhaust temperature. For example, two older IGT engines of 180 MW power could be replaced with a new IGT engine that produces 350 MW power but the turbine exhaust temperature of the newer engine would be must higher than in the older engines and thus would require a significant change to the HRSG system to be capable of handling the higher turbine exhaust temperature. In another example, two of the 300 MW IGT engines could not be replaced with a single new IGT engine because the new engine would have to produce 600 MW which does not exist at the present time.